First, and unbearably, there are the dead. Forty-nine young men and women at a gay nightclub in Orlando, Fla., doing what young men and women everywhere do on a Saturday night — singing, dancing, enjoying one another’s company.

Forty-nine more lives obliterated, 49 more families torn apart. A nation terrorized again — and again, and again, and again — by the uniquely deadly combination of twisted hatred and weapons of mass destruction as easily available as cough medicine.

Now the politics. All mass shootings convulse the nation, but this one falls in the middle of one of the nastiest, most divisive presidential campaigns in memory. And if there is anyone who might try to turn one of the worst atrocities in modern American history to his own warped ends, who could draw all the wrong lessons from the horror of what happened in Orlando, it is Donald Trump.

One can take the measure of a leader from his or her response to national trauma. So what did Mr. Trump, the presumptive Republican nominee for president, do first? He congratulated himself, on Twitter, for having predicted such an assault and called for President Obama to resign. On Monday, he said, cryptically, “Look, we’re led by a man that either is not tough, not smart or he’s got something else in mind.”

In a speech Monday afternoon, Mr. Trump doubled down on his outrageous call to bar all Muslims from entering the country. “When I’m elected I will suspend immigration from areas of the world where there’s a proven history of terrorism against the United States,” he said. “We cannot continue to allow thousands upon thousands of people to pour into our country, many of whom have the same thought process as this savage killer.”

Aside from being offensive and un-American, this would do nothing to stop people like the shooter in this case, Omar Mateen, an American citizen born in New York and living in Florida. (To evade this inconvenient fact, Mr. Trump said in his speech on Monday that Mr. Mateen was born in Afghanistan — or “Afghan,” as he put it.)

Many factors appear to have contributed to the slaughter at the Pulse nightclub, the worst mass shooting in modern United States history: a vicious and virulent homophobia; a failure to identify and intercept those with histories of domestic abuse or threats of violence; a radicalized strain of Islam that works daily to convert angry, disaffected people everywhere into mass killers.

All of these demand a response, yet none has a simple fix. One other factor, however, does. It connects the massacre in Orlando to those in Charleston, Aurora, Newtown, San Bernardino, Oak Creek and Chattanooga; at Virginia Tech and Fort Hood; and more places: easy access to guns, and especially to military-style weapons built for the sole purpose of killing scores of people in a short amount of time.

There will always be people filled with uncontrolled rage, people who want to inflict as much devastation as they can — whether in the name of a radical Islamist ideology or simple hatred of specific groups. The clear solution is to make such violence as hard as possible to commit.

But Washington’s cowardly lawmakers have only made it easier by repeatedly refusing to pass even the most obvious and least burdensome measures, like universal background checks and a strong assault weapons ban, as well as not empowering the F.B.I. to block firearms sales to people on a terror watch list. Mr. Mateen — whom the F.B.I. investigated in 2013 and 2014 for suspected terrorist ties — may well have been stopped from purchasing his guns if such laws had been in place. And he was not alone in his choice of weapon. Before the slaughter in Orlando, 95 percent of the deaths resulting from domestic terrorist attacks since Sept. 11 were inflicted by guns, according to a 2015 analysis. Radical Islamic terrorist groups openly advertise America’s lax gun laws to encourage those who want to carry out attacks here.

This is not complicated. Hillary Clinton, the presumptive Democratic nominee for president, gets it. In a speech on Monday afternoon, she said, “If the F.B.I. is watching you for a suspected terrorist link, you shouldn’t be able to just go buy a gun with no questions asked.”

Common-sense measures like these do not infringe on the Second Amendment, however much the National Rifle Association likes to pretend that the words “well regulated” have somehow disappeared from the amendment’s opening clause. Without history or logic on its side, the gun industry and its supporters instead continue to hawk only one solution: more guns, and more “good guys” carrying them, prepared to kill someone if they feel they need to. As President Obama said on Sunday, “We have to decide if that’s the kind of country we want to be.” The answer should be no.

In any case, the events of early Sunday morning once again demolished that particular Wild West fantasy. The nightclub was, in fact, protected by a good guy with a gun — an off-duty police officer, who was unable to stop the carnage.

No law will stop all gun crime, but smart laws will reduce the chance that the people at greatest risk for committing violence with guns will be able to get their hands on the deadliest weapons.

America’s gun-violence epidemic is a national public health crisis, and if it were treated like every other public health crisis, it would be studied in depth and attacked with evidence-based approaches. But Republicans in Congress have blocked that path, too, repeatedly barring federal funding into research on gun violence, because they are afraid of what it will show.

Meanwhile, the same lawmakers complain that Mr. Obama has not taken the danger of ISIS and other radical Islamic groups seriously enough. But American officials have long been alert to the threat from homegrown American terrorists, even as, in recent years, that threat has evolved as terrorist groups intensified efforts to recruit Americans for the war in Syria and for other violent acts.

The terror of gun violence is so embedded in American culture that only here must the horrific phrase “mass shooting” be preceded by the qualifier “yet another.” Today, Orlando holds the mark for the nation’s worst mass shooting. But as long as lawmakers refuse to take even the most basic steps, a new city will inevitably claim that gruesome mantle.

This inaction is as inexcusable as it is fatal. In his address to the nation on Sunday, Mr. Obama reminded Americans that “to actively do nothing is a decision.” There is another decision, however, that the voters have the power to make in November: to elect those politicians who are prepared to take on America’s gun epidemic, and to reject those who willingly tolerate it.